(1) Acronym for disk operating system. The term DOS can
refer to any operating system, but it is most often used as a shorthand
for MS-DOS (Microsoft disk operating system). Originally developed by
Microsoft for IBM, MS-DOS was the standard operating system for
IBM-compatible personal computers.
The initial versions of DOS were very simple and resembled another
operating system called CP/M. Subsequent versions have became
increasingly sophisticated as they incorporated features of minicomputer
operating systems. However, DOS is still a 16-bit operating system and
does not support multiple users or multitasking.

For some time, it has been widely acknowledged that DOS is insufficient
for modern computer applications. Microsoft Windows helped alleviate
some problems, but still, it sat on top of DOS and relied on DOS for
many services. Even Windows 95 sat on top of DOS. Newer operating
systems, such as Windows NT and OS/2 Warp, do not rely on DOS to the
same extent, although they can execute DOS-based programs. It is
expected that as these operating systems gain market share, DOS will
eventually disappear. In the meantime, Caldera, Inc. markets a version
of DOS called DR-OpenDOS that extends MS-DOS in significant ways.